In the developmental process of WO3 based transmissive ECDs, we are concentrating our efforts for developing ion storage counter electrode films with a high transmittance for visible light both in the charged and discharged state and an ion storage capacity exceeding 20 mCcm−2 or comparable to WO3 to provide sufficient number of ions for deep colouration. Therefore, work has been undertaken in this direction for the synthesis of CeO2 precursors doped with TiO2 via a wet chemistry route.
Counter electrodes in transmissive electrochromic devices (ECDs) can be of two types. The first one involves an electrochromic (EC) layer, which is complementary with the chosen electrochromic material. The combination of WO3 with NiOxHy is such typical example. The second possibility is an optically passive counter electrode, which remains colorless in both oxidized and reduced states.
Whether active or passive the counter electrode has also to balance the charge shuttled from the active EC film through the ionic conductor—the electrolyte-layer. Thus the ion storage capacity of the counter electrode should equal the ion storage capacity of EC WO3. Good cycling stability within the operational voltage and temperature range of the ECDs and high transmittance are other requirements of a counter electrode.
Among optically passive counter electrode materials In2O3: Sn has been reported as a candidate. Although it can be used also as a transparent conducting electrode its intercalation reaction is only partially reversible. Another material, which has been investigated extensively, is vanadium pentoxide (V2O5). It has high Li+ storage capacity, reversible intercalation kinetics for lithium, but the transmission in its bleached state is low. Tin oxide doped with different dopants like Mo and Sb has also been studied as a candidate for a passive counter electrode, but instability of SnO2 towards Li+ intercalation and the likely reaction leading to the formation of SnO, Sn and Li2O prompted exploratory work on other suitable materials for ECDs.
In comparison with the above-mentioned materials, CeO2 appears to be more promising optically passive counter electrode material. The reversibility of the lithium ion intercalation reaction in CeO2 is reasonably good but the reaction kinetics is very slow as shown by D. Keomany, C. Poinsignon, D. Deroo. in Sol. Energy Mater. Sol. Cells 33 (1994) 429-44. Several attempts have been made in order to improve the reaction kinetics by way of mixing the oxide with other materials such as Ti, Zr, V, Sn, Mo and Si individually or with their mixtures.
Films of pure and doped CeO2 have been made by different techniques. Sputtering technique has been adopted by M. Veszelei, L. Kullman, C. G. Granqvist, N. V. Rottkay, and M. Rubin in Appl. Opt., 37 (1998) 5993-6001 and the films thus obtained have been extensively studied. The potential of such films as passive counter electrodes has been reported. However, these authors have not obtained and reported the formation of CeTi2O6 compound in the thin film form. It is noteworthy that the application of CeO2—TiO2 mixed oxide films as passive counter electrodes has been shown by several authors. However, the potential of CeTi2O6 thin films as passive counter electrodes has never been reported earlier. Therefore, the present invention is the first report on the application of these films as passive counter electrodes for electrochromic devices.
The widely used sol-gel process offers numerous advantages over the other conventional deposition techniques, which include tailor making of the film's properties, introduction of porosity in the films, low process cost and possible processing at low temperature. Preparation of CeO2 based films by sol-gel technique has been attempted following various routes. The use of alkoxides, the most popular precursor material in sol-gel processing has been reported by D. Keomany, C. Poinsignon, D. Deroo. in Sol. Energy Mater. Sol. Cells 33 (1994) 429-441. Alternately salts of cerium like CeCl3.7H2O, [(NH4)2{Ce(NO3)6}] in combination with Ti alkoxides have been shown by A. Makishima, M. Asami and K. Wada, in J. Non-Cryst. Solids 121 (1990) 310-314 as one of the routes to get CeO2—TiO2 films. Based on the earlier reports on these materials, A. Makishima, M. Asami and K. Wada, in J. Non-Cryst. Solids 121 (1990) 310-314 have performed a study in which the type of alkoxyl group of titanium alkoxide and the kind of catalyst have been varied in order to study their influence on the properties of the films. The films deposited by the authors have been annealed at 500° C. and the XRD patterns of the films are characterized by the appearance of diffraction peaks of the CeO2 phase alone.
Metal oxide semiconductors are used for gas sensing due to the dependence of their electrical conductivity on the ambient gas composition. They offer the possibility of “tailoring” the sensitivity and selectivity towards specific gas species. Gaining increased attention are mixed metal oxide compounds whereby varying the composition of the constituents, the sensor performance can be modified, i.e. improvement of sensitivity and selectivity, fabrication of n- and/or p-type semiconductor and modification of the sensor resistance for ease of electronic interface. Because of its chemical stability and high diffusion coefficient of oxygen vacancies, CeO2 is a promising material for fast oxygen gas sensing at high temperatures as reported by F. Millot, De Mierry in J. Phys. Chem. Solids 46 (1985) 797-801. TiO2 has been widely reported by A. Rothschild, F. Edelman, Y. Komen, F. Cosandey, in Sensors and Actuators B 67 (2000) 282-289, N. O. Savage, S. A. Akbar, P. K. Dutta, in Sensors and Actuators B 72 (2001) 239-248 and C. Garzella, E. Comini, E. Tempesti, C. Frigeri, G. Sberveglieri in Sensors and Actuators B 68 (2000) 189-196 for its gas sensing properties towards oxygen, carbon monoxide, methanol and ethanol and humidity. Mixed CeO2—TiO2 films deposited using a sol-gel process involving ceric ammonium nitrate and titanium butoxide have been reported to be oxygen gas sensors by A. Trinchi, Y. X. Li, W. Wlodarski, S. Kaciulis, L. Pandolfi, S. Viticoli, E. Comini, G. Sberveglieri in Sensors and Actuators B 95 (2003) 145-150.
Photocatalytic reaction sensitized by TiO2 and other semiconducting materials has attracted extensive interest as a potential way of solving energy and environmental issues. Several cerium titanates in the powdered form have been investigated for photocatalytic activity. Yellow colored cerium titanate, CeTi2O6 with mainly Ce4+ state is known to cause photobleaching of methylene blue aqueous solution with irradiation of Xe discharge light as reported by S. O-Y-Matsuo, T. Omata, M. Yoshimura in J. Alloys and Compounds, 376 (2004) 262-267. Mixed CeO2—TiO2 films deposited using R.F and D.C sputtering are reported by Q. N. Zhao, C. L. Li, X. J. Zhao in Key Engineering Materials 249 (2003) 451-456 to decolorize methyl orange solutions upon irradiation of the UV light.
Brannerite, UTi2O6 is an accessory phase in the titanate-based crystalline ceramics of synroc as reported by A. E. Ringwood, S. E. Kession, N. G. Ware, W. Hibberson, A. Major in Nature (London) 278 (1979) 219. The high U-content of brannerite (up to 62.8 wt. %) and its potential as a nuclear waste form for the immobilization of actinides have emphasized the importance of understanding radiation damage effects and their relation to composition and structure. The ideal formula of natural brannerite is (U,Th)1−xTi2+xO6 with a deficiency in uranium and excess titanium. Many cation substitutions have been identified for both uranium (Pb, Ca, Th, Y and Ce) and titanium (Si, Al, Fe) in natural brannerite. Ideally, stoichiometric brannerite is monoclinic with space group C2/m. There are two different distorted octahedra in the structure. Distorted TiO6 octahedra form a zigzag sheet by sharing common edges, and each Ti octahedron shares three edges with titanium octahedral and three corners with uranium octahedra. The sheets of TiO6 octahedral are identical with those of the anatase structure parallel to (101) plane. The large cations (Th,U) are located at the interlayer sites and connect adjacent sheets. Each uranium octahedron shares two common edges with neighboring UO6 octahedra and four corners with TiO6 octahedra. Oxygen atoms exist in a distorted HCP (hexagonal closely packed) array. Ce can substitute onto the U-site with little distortion of the octahedra. Ce is commonly used to estimate the properties of solids containing plutonium owing to their similar ionic radii (Ce(IV)=0.087 nm; Pu(IV=0.086 nm)). The compound CeTi2O6 is isostructural with PuTi2O6. As has been reported by K. B. Helean, A. Navrotsky, G. R. Lumpkin, M. Colella, J. Lian, R. C. Ewing, B. Ebbinghaus and J. G. Catalano in J. Nucl. Mater. 320 (2003) 231-244, CeTi2O6 in the powdered form prepared by sintering in air at 1350° C. for>100 h a pellet containing stoichiometric portions of the oxides, CeO2 and TiO2 is used to estimate the properties of PuTi2O6.
In the present invention, the CeTi2O6 phase has been achieved in thin film by the sol-gel technique, which represents a reliable, low-cost chemical route. In comparison to the powdered CeTi2O6 material, which is formed at 1400-1500° C., the corresponding thin film is prepared by the sol-gel process at 600° C. in the present invention. Our literature survey has shown that the preparation of CeTi2O6 in thin film form has never been carried out before. CeTi2O6 in powdered form has applications in areas of e.g. immobilization of nuclear waste form and photocatalytic activity. However, the same material in thin film form is useful in applications such as passive counter electrodes, sensors and photocatalytic activity. We have prepared CeTi2O6 films with a high transmittance for visible light both in the charged and discharged state and an ion storage capacity exceeding 20 mCcm−2. Using cerium chloride heptahydrate and titanium propoxide precursors, we have reported earlier in Sol. Ener. Mater. Sol. Cells 86 (2005) 85-103, the formation of a mixed compound of CeO2 and TiO2 i.e. CeO1.6.2TiO2 at annealing temperature of 500° C. from the Ce:Ti compositions, 4:1 and 2:1. It is evident from the chemical formula of the aforesaid compound that the oxygen content and thus the stoichiometric compositions of the two compounds i.e. CeO1.6.2TiO2 and CeTi2O6 are different. A comparative chart showing the results reported in the published papers and the present patent is given in Table I.
TABLE IA comparative chart showing the process steps, products obtained and the application of the films derived fromdifferent Ce:Ti compositions.Ce:TimoleratioProcess stepsProduct(s) obtainedApplication(s)  4:1a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Prior-art)As a passiveabsolute ethanol;CeO2 and CeO1.6•2TiO2counterb)adding the solution prepared in step a) to titanium propoxide to obtain a(crystallite size: 6.2 nm)electrode.final solution containing Ce:Ti mole ratio of 4:1;{A. Verma, S. B. Samanta,c)stirring the mixture obtained in b);N. C. Mehra,d)after aging the solution as obtained in c) for a period of one week, spinA. K. Bakhshi, S. A.coating the solution on electrically conducting substrates and micro slideAgnihotry in Sol. Ener.glass at 3000 rpm for a duration of 35 s;Mater. Sol. Cells 86e)after initially drying the film deposited in d) for 5 min. annealed the film in(2005) 85-103,air at 500° C.A. Verma, Amish G. Joshi,A. K. Bakhshi,S. M. Shivaprasad, S. A.Agnihotry in Appl. Surf.Sci. (article in press)}.  2:1a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Prior-art)As a passiveabsolute ethanol;CeO2 and CeO1.6•2TiO2counterb)adding the solution prepared in step a) to titanium propoxide to obtain a(crystallite size: 9.2 nm)electrode.final solution containing Ce:Ti mole ratio of 2:1;{A. Verma, S. B. Samanta,c)stirring the mixture obtained in b);N. C. Mehra,d)after aging the solution as obtained in c) for a period of one week, spinA. K. Bakhshi, S. A.coating the solution on electrically conducting substrates and micro slideAgnihotry in Sol. Ener.glass at 3000 rpm for a duration of 35 s;Mater. Sol. Cells 86e)after initially drying the film deposited in d) for 5 min. annealed the film in(2005) 85-103,air at 500° C.A. Verma, Amish G. Joshi,A. K. Bakhshi,S. M. Shivaprasad, S. A.Agnihotry in Appl. Surf.Sci. (article in press)}.1.33:1 a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Prior-art)As a passiveabsolute ethanol;CeO2 and CeO1.6•2TiO2counterb)adding the solution prepared in step a) to titanium propoxide to obtain a{A. Verma, S. B. Samanta,electrode.final solution containing Ce:Ti mole ratio of 1.33:1;N. C. Mehra,c)stirring the mixture obtained in b);A. K. Bakhshi, S. A.d)after aging the solution as obtained in c) for a period of one week, spinAgnihotry in Sol. Ener.coating the solution on electrically conducting substrates and micro slideMater. Sol. Cells 86glass at 3000 rpm for a duration of 35 s;(2005) 85-103}.e)after initially drying the film deposited in d) for 5 min. annealed the film inair at 500° C.  1:1a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Prior-art)As a passiveabsolute ethanol;Amorphouscounterb)adding the solution prepared in step a) to titanium propoxide to obtain a{A. Verma, S. B. Samanta,electrode.final solution containing Ce:Ti mole ratio of 1:1;N. C. Mehra,c)stirring the mixture obtained in b);A. K. Bakhshi, S. A.d)after aging the solution as obtained in c) for a period of one week, spinAgnihotry in Sol. Ener.coating the solution on electrically conducting substrates and micro slideMater. Sol. Cells 86glass at 3000 rpm for a duration of 35 s;(2005) 85-103,e)after initially drying the film deposited in d) for 5 min. annealed the film inA. Verma, Amish G. Joshi,air at 500° C.A. K. Bakhshi,S. M. Shivaprasad, S. A.Agnihotry in Appl. Surf.Sci. (article in press)}.0.33:1 a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Prior-art)As a passiveabsolute ethanol;Amorphouscounterb)adding the solution prepared in step a) to titanium propoxide to obtain a{A. Verma, Amish G. Joshi,electrode.final solution containing Ce:Ti mole ratio of 0.33:1;A. K. Bakhshi,c)stirring the mixture obtained in b);S. M. Shivaprasad, S. A.d)after aging the solution as obtained in c) for a period of one week, spinAgnihotry in Appl. Surf.coating the solution on electrically conducting substrates and micro slideSci. (article in press)}.glass at 3000 rpm for a duration of 35 s;e)after initially drying the film deposited in d) for 5 min. annealed the film inair at 500° C.0.6:1a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Present invention)a) As a passiveabsolute ethanol;a) Amorphous atcounterb)adding the solution prepared in step a) to titanium propoxide to obtain aannealingelectrode uponfinal solution containing Ce:Ti mole ratio of 0.6:1;temperature ofannealing atc)stirring the mixture obtained in b) for a period of 5 min;500° C.500° C.d)after aging the solution as obtained in c) for a period of one week, spinb) CeTi2O6 phase atb) As passivecoating the solution on electrically conducting substrates and micro slideannealingcounterglass at 3000 rpm for a duration of 35 s;temperature ofelectrode,e)after initially drying the film deposited in d) for 5 and 15 min, annealed600° C.sensor, and athe film in air at 500 and 600° C. respectively.photocatalyticagent upon theformation ofCeTi2O6 phaseat annealingtemperature of600° C.0.5:1a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Present invention)a) As a passiveabsolute ethanol;a) Amorphous atcounterb)adding the solution prepared in step a) to titanium propoxide to obtain aannealingelectrode uponfinal solution containing Ce:Ti mole ratio of 0.5:1;temperature ofannealing atc)stirring the mixture obtained in b) for a period of 5 min;500° C.500° C.d)after aging the solution as obtained in c) for a period of one week, spinb) CeTi2O6 phase atb) As passivecoating the solution on electrically conducting substrates and micro slideannealingcounterglass at 3000 rpm for a duration of 35 s;temperature ofelectrode,e)after initially drying the film deposited in d) for 5 and 15 min, annealed600° C.sensor, and athe film in air at 500 and 600° C. respectively.photocatalyticagent upon theformation ofCeTi2O6 phaseat annealingtemperature of600° C.0.4:1a)preparing an 0.22 M alcoholic solution of cerium chloride heptahydrate in(Present invention)a) As a passiveabsolute ethanol;a) Amorphous atcounterb)adding the solution prepared in step a) to titanium propoxide to obtain aannealingelectrode uponfinal solution containing Ce:Ti mole ratio of 0.4:1;temperature ofannealing atc)stirring the mixture obtained in b) for a period of 5 min;500° C.500° C.d)after aging the solution as obtained in c) for a period of one week, spinb) CeTi2O6 phase atb) As passivecoating the solution on electrically conducting substrates and micro slideannealingcounterglass at 3000 rpm for a duration of 35 s;temperature ofelectrode,e)after initially drying the film deposited in d) for 5 and 15 min, annealed600° C.sensor, and athe film in air at 500 and 600° C. respectively.photocatalyticagent upon theformation ofCeTi2O6 phaseat annealingtemperature of600° C.